Method and system for determining the orientation of magnetic fields by means of GMR sensors

ABSTRACT

The invention relates to a method and a system for determining the orientation of an external magnetic field by means of giant magneto-resistor (GMR) sensors. A sensor circuit is provided with two or more sub-circuits each having two GMR sensors connected in series. Three respective sensor voltages are obtained and a scaling factor is determined from differences in these voltages. An external magnetic field orientation is determined based on a scaling factor applied to the difference voltages. The inventive method enables signals from the GRM sensors to be reprocessed easily in order to compensate the temperature dependence of the signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to a method for determining the orientation ofan external magnetic field with giant magneto-resistor (GMR) sensors.The invention is also directed to a system with GMR sensors fordetermining the orientation of an external magnetic field.

2. Description of the Related Art

GMR sensors were first manufactured in the 1980's. They aredistinguished by their high sensitivity of their electrical specificresistance to the orientation of an external magnetic field.

GMR sensors have proven themselves as ideal sensors for non-contactingposition and speed measurement of magnetic objects and for thenon-contacting measurement of electrical currents. Due to theirruggedness when confronted by harsh environmental influences, they areoften utilized in an industrial setting or in the field of automotiveelectronics. Since the resistance of the GMR sensors is dependent on theorientation of the external magnetic field and, within a broad range,not on the strength of the magnetic field, the GMR sensor can be easilyemployed for measuring field orientations without the distance betweenthe magnetic source and the GMR sensor having to be exactly adjusted.

There are various embodiments of GMR sensors. FIG. 1 illustrates anembodiment of a GMR sensor illustrated that is composed of very thinanti-ferromagnetic layers that are alternately stacked with very thin,conductive, non-magnetic layers 1-2, for example copper. The uppermostand the bottom layer, i.e., the covering layers 1-1 of the GMR sensor,are composed of a soft anti-ferromagnetic material whose magneticorientation already aligns in accord with soft, external magnetic fields1-5 that penetrate the material. The other layers lying between theconductive, non-magnetic layers 1-2 are hard anti-ferromagnetic layers1-3 that, for example, are composed of cobalt. A magnetic direction canbe impressed in hard anti-ferromagnetic layers using a very strongexternal field, this direction not changing even given moderately strongexternal fields with a different orientation. The orientation of theanti-ferromagnetic, hard layers 1-3 is impressed once with a very strongexternal magnetic field (>15 kA/m) and determines the orientation of theinternal magnetic field of the GMR sensor and, thus, the internalorientation of the GMR sensor. In order to nonetheless change theinternal orientation of the GMR sensors, an external magnetic field ofat least the same strength must be applied again.

The variation of the electrical resistance due to a variation of theorientation of an external magnetic (soft) field is based on the factthat the resistance that electrons encounter in magnetic materials isdependent on the angle between spin directions of the electrons and themagnetic field direction of the material. Electrons that have assumedthe spin direction of the external magnetic field 1-5 in the softcovering layer 1-1 encounter a low electrical resistance in the hardmagnetic layers 1-3 when the internal magnetic field 1-6 of the GMRsensor has the same magnetic orientation as the external magnetic field1-5. The electrical combined resistance, R, of a GMR sensor is thuscomposed of a fundamental resistance R0 and an angle-dependentresistance R1=½ΔR(1−cos α), so that the following equation applies:R(T·α)=R 0(T)+½ΔR(T)×(1−cos α).  (1)

-   -   where α is the angle between internal orientation 1-6 of the GMR        sensor and the orientation of the external field 1-5 in the        plane of the covering layers 1-1, and ΔR(T) is the maximum        magneto-resistive resistance of the GMR sensor.

The parameter T incorporated in Equation (1) indicates that theresistance is somewhat dependent on the temperature of the GMR sensor.Typically, the fundamental resistance increases given increasingtemperature, whereas the magneto-resistive resistance drops with thetemperature. Typical values for the relative temperature drift of thefundamental resistance R0(T) are approximately 0.05%/K through 0.2%/Kand approximately −0.05%/K through −0.2%/K for the relative temperaturedrift of the magneto-resistive resistance. This temperature dependencycauses an angle allocation from a measured resistance to be ambiguouswhen the temperature varies and is unknown.

Typically, the layers of the GMR sensors are only a few nm thick and areapplied onto small substrates with precision techniques, for examplesputtering methods. The structures of the GMR sensors are applied onto asubstrate with lithographic methods so that a plurality of GMR sensorscan also be applied onto the substrate in a small space. The electricalconnections between the GMR sensors are often designed in a meanderingfashion in order to generate defined resistances on the leads to the GMRsensors.

FIG. 2 shows a simple measuring structure with a GMR sensor of the PriorArt with which the orientation of an external magnetic field can beidentified in the plane of the GMR sensor layers without temperaturecompensation. The measuring structure is composed of a current source2-1 with a set current I, of a GMR sensor 2-2 with an internalorientation 2-5, as well as of a voltmeter device 2-3 that takes thevoltage drop-off V=V(P2)−V(P1) across the GMR sensor at the points P1and P2. When R0 and ΔR of the GMR sensor are known from precedingcalibration measurements, then the angle α between external magneticfield 2-4 and internal orientation 2-5 can be calculated from themeasured voltage drop-off with the assistance of Eq. (1). This simplemeasuring structure, however, has the following disadvantages:

-   -   1) the angle measurement is unambiguous only in the region of        180 degrees but not in the full 360 degree range;    -   2) the magneto-resistive effect, ΔR/R0, generally amounts to        only a few percent, so that the voltage measurement is        superimposed by a large offset voltage V_(offs)=R0×I; and    -   3) the temperature dependency of the parameters R0(T) and ΔR(T)        does not allow an unambiguous angle determination given an        unknown temperature of the GMR sensor.

A significant improvement for avoiding these difficulties is representedby the employment of two series-connected, structurally identical GMRsensors with internal orientation anti-parallel to one another (FIG. 3 aand FIG. 3 b). FIG. 3 a shows the two GMR sensors 3-2 and 3-3 withanti-parallel internal orientation and the current source 3-1 with theset current I₁ that flows off to the current sink 3-6 via the points P3,P2 and P1. The external magnetic field 3-4 acting on the GMR sensorsdescribes the angle α with the internal magnetic orientation of the GMRsensor 3-2. The external magnetic field should be the same for both GMRsensors, which is easily met when the spatial extent of the two GMRsensors together is small compared to the spatial structure of theexternal magnetic field 3-4.

When the anti-parallelism of the GMR sensor 3-3 to the reference GMRsensor 3-2 is taken into consideration, then the following derives fromEq. (1) and FIG. 3 a:V(P 2)−V(P 1)=(R 0(T)+½ΔR(T)×(1+cos α))×I ₁  (2)andV(P 3)−V(P 2)=(R 0(T)+½ΔR(T)×(1+cos α))×I ₁  (3)

The difference V1 of the two voltages then supplies:V 1=V(P 2)−V(P 1)−V(P 3)+V(P 2)=ΔR(T)×I ₁×cos α  (4)

The employment of two series-connected anti-parallel GMR sensors canthus be utilized by difference formation to eliminate the great (andtemperature-dependent) voltage offset R0(T) that occurs in FIG. 2. Thedifference formation can be realized in a simple way with analogelectronic circuits to prevent time delays that occur due todigitalization and digital calculations.

In order to eliminate the ambiguity of the angle determination in the360 degree range, a second circuit of the type of FIG. 3 a may be drivenin parallel with the same current source. Such a second circuit is shownin FIG. 3 b. Such electronic circuits with GMR sensors for themeasurement of the orientation of a magnetic field can, for example, bederived from the German patent document DE 196 19 806. The secondcircuit comprises GMR sensors 3-10 and 3-11 with the same structure asthe first circuit, with the difference that the internal orientations ofthe two GMR sensors are perpendicular to the farthest-reaching extent tothe internal orientations of the GMR sensors of the first circuit. Thetwo circuit must lie close enough together that they essentiallyencounter the same external magnetic field 3-4. The current source 3-12generates the current I₂ that flows off to the current sink 3-13 via thepoints P3′, P2′ and P1′. The voltages V(P2′)−V(P1′) and V(P3′)−V(P2′)that form at the points P1′, P2′ and P3′ are subtracted from one anotheras in FIG. 3 a, so that—analogous to Eq. (4)—a second angle-dependentvoltage measurement is obtained withV 2=V(P 2′)−V(P′)−V(P 3′)+V(P 2′)=ΔR(T)×I ₂×sin α.  (5)

By division of Eq. (5) by Eq. (4), a temperature-independentrelationship is obtained between the angle α and the measured voltagesV1, V2, V1′ and V2′:α=arctan(ΔV′/ΔV)=arctan((V 1′−V 2′)/(V 1−V 2)),  (6)

-   -   where the currents I₁ and I₂ are assumed to be the same for the        sake of simplicity. In order to assure the unambiguousness in        the entire 360 degree angle determination, a circuit must be        additionally utilized that decides on the basis of V1, V2, V1′        and V2′ whether the angle is situated in the range between        −π/2<α<+π/2 or in the range −π<α<−π/2 or, respectively,        +π/2<α<+π.

FIG. 4 shows such an electronic circuit of the Prior Art upon employmentof the GMR sensor circuits described in FIG. 3 a and FIG. 3 b, thesebeing connected in parallel to one another, i.e. P1=P1′ and P3=P3′. Thefirst GMR sensor 4-4 and the series-connected, second GMR sensor 4-5,both of which have anti-parallel internal magnetic orientation relativeto one another, form the first sensor sub-circuit 4-2; the first GMRsensor 4-6 and the series-connected, second GMR sensor 4-7, whichlikewise have anti-parallel internal magnetic orientation, form thesecond sensor sub-circuit 4-3. The internal magnetic orientation of thefirst sensor sub-circuit 4-2, which is established by the internalmagnetic orientation of the first GMR sensor 4-4, is also vertical tothe internal magnetic orientation of the second sensor sub-circuit,which is established by the internal magnetic orientation of the firstGMR sensor 4-4. The orientation of the external magnetic field 4-14describes the angle α with the orientation of the internal magneticfield of the first GMR sensor 4-4 of the first sensor sub-circuit 4-2.

Both sensor sub-circuits are supplied by the same current source 4-1.Since the four GMR sensors have essentially the same resistanceparameters, the current divides equally onto the two sensorsub-circuits. The two operational amplifiers 4-10 and 4-11 form thedifference between the signals V(P2)−V(P4) and V2=V(P2′)−V(P4), so thatthe output signal is freed of offset in the way described above:V 1=V(P 2)−V(P 4)=½ΔR(T)cos α×I/2  (7)andV 2=V(P 2′)−V(P 4)=½ΔR(T)sin α×I/2  (8).

The ratio V2/V1 supplies an unambiguous, temperature-independentrelationship between angle α and the two measured output signals:α=arctan(V 2/V 1)  (9).

One disadvantage of the relationship indicated in Eq. (6) and Equ. (8),however, is that the calculation of the arctan cannot be accomplishedwith a simple analog circuit. An approximate linearization is also onlypossible in a small value range. The calculation therefore generallyrequires a digitalization of the two output signals with a subsequentcalculation of the arctan. The digitalization of the two values,however, is both cost-intensive as well as time-consuming. The digitalcalculation of the arctan also requires programming, since this functionis not implemented in standard micro-controller circuits. This alsoleads to time delays. Finally, the tangent exhibit infinities in thevalue range that are handled poorly with a computer.

SUMMARY OF THE INVENTION

The invention is therefore based on the object of providing a methodthat eliminates or at least reduces the described problems in a simpleand fast way particularly the temperature-dependent drift that theoutput signals of the two series-connected anti-parallel GMR sensorpairs experience. The invention is also based on the object of offeringa system with GMR sensors for determining the orientation of an externalmagnetic field that calculates the temperature drift of the outputsignals of the two series-connected anti-parallel GMR sensor pairs outin a simple and fast way.

This object is achieved by a method for determining the orientation ofan external magnetic field. Inventively, the method for determining theorientation of an external magnetic field with GMR sensors is providedas follows:

-   -   a) a sensor circuit having at least a first and a second sensor        sub-circuit that are connected parallel to one another is        offered, whereby the first and the second sensor sub-circuit        each respectively comprise at least a first and a second GMR        sensor that are connected in series and that are respectively        arranged in opposite internal magnetic orientation, whereby the        internal magnetic orientation of the GMR sensors of the first        sensor sub-circuit is essentially perpendicular to the internal        magnetic orientation of the GMR sensors of the second sensor        sub-circuit;    -   b) respectively three sensor voltages V(P1), V(P2), V(P3) are        taken at the points P1, P2 and P3 in the first and the second        sensor sub-circuit, whereby P1 lies on the connecting line from        the current sink to the second GMR sensor, P2 lies on the        connecting line from the second GMR sensor to the first GMR        sensor and P3 lies on the connecting line from the first GMR        sensor to the current supply;    -   c) a first difference circuit generates a difference voltage V1        from the three sensor voltages of the first sensor sub-circuit,        and a second difference circuit generates a difference voltage        V2 from the three sensor voltages of the second sensor        sub-circuit;    -   d) a scaling factor is calculated with the assistance of V1 and        V2;    -   e) V1 and V2 are scaled such with the scaling factor that the        orientation of the external magnetic field can be determined        from them.

An inventive system with GMR sensors for determining the orientation ofan external magnetic field is also provided, with the followingfeatures:

-   -   a) the system comprises a sensor circuit having a first and a        second sensor sub-circuit in which the first sensor sub-circuit        is connected parallel to the second sensor sub-circuit and the        first and the second sensor sub-circuit each respectively        comprise at least a first and a second GMR sensor that are        connected in series and that are respectively arranged in        opposite internal magnetic orientation relative to one another,        and in which the internal magnetic orientation of the GMR        sensors of the first sensor sub-circuit is essentially        perpendicular to the internal magnetic orientation of the GMR        sensors of the second sensor sub-circuit;    -   b) the first and the second sensor sub-circuit respectively        comprise three measuring points P1, P2 and P3, where P1 lies on        the connecting line from the current sink to the second GMR        sensor, P2 lies on the connecting line from the second GMR        sensor to the first GMR sensor and P3 lies on the connecting        line from the first GMR sensor to the current supply, so that        the three sensor voltages V(P1), V(P2), V(P3) can be taken at        the measuring points P1, P2 and P3;    -   c) the system comprises a first and a second difference circuit,        in which the first difference circuit generates a difference        voltage V1 from the three sensor voltages of the first sensor        sub-circuit, and a second difference circuit generates a        difference voltage V2 from the three sensor voltages of the        second sensor sub-circuit; and    -   d) the system comprises a scaling circuit that reads out the        difference voltage V1 and the difference voltage V2 and scales        them such that the orientation of the external magnetic field        can be determined from the scaled values.

These inventive embodiments, along with various alternate embodiments,are discussed more fully below.

DESCRIPTION OF THE DRAWINGS

The invention is presented in greater detail below on the basis of FIGS.5 and 6.

FIG. 1 is a pictorial isometric diagram of a GMR sensor of the PriorArt;

FIG. 2 is a schematic of a basic circuit with a GMR sensor for measuringthe orientation of an external magnetic field according to the PriorArt;

FIGS. 3 a-3 b are schematic diagrams of sensor sub-circuits withrespectively two GMR sensors for the sensor circuit of the Prior Art;

FIG. 4 is a schematic diagram of a sensor circuit with GMR sensors and adifference circuit for determining the orientation of an externalmagnetic field according to the Prior Art;

FIG. 5 is a graph illustrating the inventive method for determining theorientation of an external magnetic field; and

FIG. 6 is a schematic block diagram for inventive embodiment of theelectronic circuit with GMR sensors for determining the orientation ofan external magnetic field.

DETAILED DESCRIPTION OF THE INVENTION

With the assistance of four GMR sensors, the inventive method makes itpossible to determine the orientation of an external magnetic field thatis established by the angle α, where the result istemperature-independent because of the scaling, and where the use ofarctan and/or arccotan functions can be foregone in the calculation. Theuse of arctan and/or arccotan functions for calculating the angle αrequires a digitalization of the measured values and necessitates theuse of programmed microprocessors, which is costly andcalculation-intensive. The inventive method, in contrast, only requiresthe calculation of a scaling factor, that is preferably composed of thesquare root of the square sum of V1 and V2 or of approximations to thissquare root. Furthermore, the inventive method largely compensates fornegative effects of the unit scatter of the participating GMR sensors.

The scaling with the scaling factor can preferably be realized withanalog circuits. The advantage of analog processing of the measuredvalues is comprised in the simplicity and in the superior speed withwhich the measured values are edited, since time delays due to theanalog/digital conversion are sidestepped and the calculating time dueto the digital signal processing is avoided.

The orientation of an external magnetic field is established by theangle α between the direction of the external magnetic field and theorientation of the sensor circuit. By definition, let the orientation ofthe sensor circuit be established below by the internal orientation ofthe first sensor sub-circuit, where the internal orientation of thefirst sensor sub-circuit is defined by the internal magnetic orientationof the first GMR sensor. Fundamentally, the orientation of the sensorcircuit can also be defined by some other GMR sensor, which changes theinventive method only insignificantly. The addition of an angle constantto the measured value is merely required at the end of a measurement.

The external magnetic field whose orientation is to be identified shouldpreferably exhibit a minimum strength so that the GMR sensor responds.On the other hand, the magnetic strength should not be so large that theinternal magnetic orientation that is impressed on every functional GMRsensor is not lost. Typical values for fields that can be measured withGMR sensors lie in the range of 5-10 kA/m. Since the GMR sensors areonly sensitive to magnetic field components parallel to the GMR layerplanes, this field strength particularly refers to the field componentsin this plane.

Furthermore, the spatial structure of the external magnetic field shouldpreferably be greater than the region in which the four GMR sensors areaccommodated. This assures that the four GMR sensors encounter the sameexternal magnetic field to the farthest-reaching extent, and the outputvoltage V1 and V2 can be exactly scaled.

There are various GMR sensor types that come into consideration for thesensor circuit. It is critical for the inventive method that the GMRsensors respectively comprise a (hard) internal magnetic field with apermanently impressed magnetic orientation, where the dependency of theGMR sensor resistance on the direction of the external magnetic fieldmust be largely described by Eq. (1). Preferably, the parameters R0(T)and ΔR(T) of the GMR sensors installed in the sensor circuit areidentical, or at least very similar, so that the temperature dependencycan be compensated as completely as possible by the inventive method.

The GMR sensors of a circuit are preferably of the same type and arepreferably applied on a common substrate so that the curve of theparameters R0(T) and ΔR(T) and the prevailing temperature T areoptimally the same for all GMR sensors. It is also advantageous when theGMR sensors are as small as possible and lie close to one another sothat the overall expanse of the GMR sensor region is small and the GMRsensors thus encounter the same external magnetic field strengthsinsofar as possible.

The sensor circuit preferably comprises two sensor sub-circuits, each ofwhich has at least two GMR sensors. However, more than two sensorsub-circuits are also conceivable, for example, for redundantmeasurements. A sensor sub-circuit respectively comprises at least afirst and a second GMR sensor that are connected in series with oneanother and that respectively have an largely anti-parallel internalmagnetic orientation relative to one another. However, a sensorsub-circuit can also comprise more than two GMR sensors orientedanti-parallel relative to one another; for example, each GMR sensor canbe replaced by two or more GMR sensors oriented parallel to one another.A sensor sub-circuit also preferably comprises series resistors withwhich, for example, a minimum overall resistance of the sensorsub-circuit can be achieved. This resistance is preferably generated byfashioning meandering lines between the GMR sensors.

The first and second difference circuit respectively generate adifference voltage V1 and V2 from the three sensor voltages V(P1), V(P2)and V(P3). Since the internal orientation of the first sensorsub-circuit is largely perpendicular to the internal orientation of thesecond sensor sub-circuit, the first difference circuit generates adifference voltage V1 that essentially represents the cosine parts ofthe orientation of the external magnetic field, whereas the seconddifference circuit generates a difference voltage V2 that essentiallyrepresents the sine parts of the orientation of the external magneticfield. The perpendicular internal orientation of the two sensorsub-circuits relative to one another thus enables the inventivecalculation of the scaling factors. What is meant by “perpendicular” isthat the angle between the internal orientations of the first sensorsub-circuit and the second sensor sub-circuit can amount both toapproximately 90 degrees as well as approximately 270 degrees.

The difference voltages V1 and/or V2 are preferably conducted to ascaling circuit that preferably calculates scaling factors with bothvalues and thus undertakes the scaling of V1 and V2. The orientation ofthe external magnetic field can be identified temperature-independentlyfrom the two scaled V1 and V2 values.

A precise acquisition of the orientation of the external magnetic fieldswith the assistance of two inventive sensor sub-circuits is alsodependent on the constancy of the ratio of the currents through the twosensor sub-circuits. The current through the first sensor sub-circuitand the current through the second sensor sub-circuit are preferablylargely the same. In this way, the cosine and the sine parts of theorientation of the external magnetic field are identically weighted, sothat the scaling can be correctly implemented given identical differencecircuits. Preferably, the equality of the currents is realized in thatboth sensor sub-circuits exhibit the same resistance coefficients R0(T)and ΔR(T) and both sensor sub-circuits are supplied by a common currentsource or voltage source so that the currents of both sensorsub-circuits are the same based on Ohm's law even given a varyingtemperature.

When the two currents are constant but not the same, then a subsequent,compensating weighting of the cosine parts must be implemented, thisbeing preferably realized by different gain factors of the differencecircuits.

The scaling is preferably calculated using the square root of the squaresum V1 and V2. This is the value with which V1 and/or V2 must be scaledin order to obtain the orientation α of the external magnetic fieldindependently of temperature. Preferably, both V1 as well as V2 arescaled in order to unambiguously identify α on the entire 360 degreerange.

The calculation of the scaling factor preferably ensues with an analogcircuit since time delay and costs due to the digitalization of the V1and V2 values can thus be avoided. The digital calculating time for thecalculation of the root of the square sum of V1 and V2 is alsoeliminated. Preferably, the squaring of the V1 and V2 values isimplemented parallel with two analog multiplier circuits. The sum of thetwo squared values is preferably generated in an analog fashion in afurther circuit. Finally, the root of the square sum of V1 and V2 isalso preferably generated in analog fashion. When only a small angularrange is considered for the calculation of α or when only a limitedprecision of the angle determination is required, then an analogapproximation is implemented instead of the square root function,preferably also a constant approximation value, a linear approximationand/or approximations of a higher order. The equation SQRT(x²+y²)=0.96×→0.398y with x>y can preferably be employed as a linearapproximation.

The root operation can also preferably be digitally implemented, which,despite digitalization, proceeds faster and with less outlay (programmedmicroprocessor, etc.) than the digitalization and calculation of thearctan or arccotan functions as implemented in methods of the Prior Artfor calculating the orientation of an external magnetic field.

The scaling of the V1 and/or V2 values preferably ensues by division ofthe V1 and/or V2 values by the calculated scaling factor. The divisionpreferably ensues in an analog way in order to eliminate time delaysduring the digitalization and circuit outlay. The division isunproblematical, particularly because the scaling factor is alwaysgreater than 0.

The determination of the orientation angle of the external magneticfield, α, from the scaled V1 and/or V2 values can ensue in various ways.In a preferred version, the arc cosine and/or arc sine is applied toscaled V1 and/or V2 values, so that the following applies:α=arccos(V 1) and/or a α=arcsin(V 2)  (10).

It is disadvantageous that these functions require a greater circuitoutlay, including programmed microprocessor and digitalization. Incontrast to the arc cotangent and arc tangent, however, these functionshave no infinities in the value range.

In another preferred embodiment, there is a table that allocates anangle α to each V1 and/or V2 value. This table can be taken from acalibration measurement or—preferably—from a one-time calculation. Sincethe scaled V1 and V2 values are temperature-independent, only one tableat a particular temperature is required.

The inventive system with sensors, difference circuit and scalingcircuit enables a simple acquisition and calculation of the orientationof an external magnetic field, by which the calculation can beimplemented completely or largely in analog.

The simplicity of the temperature-compensated calculation of theorientation of an external magnetic field is based on the magneticallyanti-parallel arrangement of the GMR sensors in the respective sensorsub-circuits and on the magnetically perpendicular arrangement of thesensor sub-circuits relative to one another. The simplicity of thetemperature-compensated calculation is also based on the constancy ofthe ratio of the currents that flow through the two sensor sub-circuits.Preferably, the current through the first sensor sub-circuit and thecurrent through the second sensor sub-circuit are the same. Bothcurrents are preferably taken from a common current source, where theoverall resistance of the first sensor sub-circuit and of the secondsensor sub-circuit are largely identical.

The first and the second difference circuit are preferably identical,particularly when the currents through the two sensor sub-circuits arethe same. Preferably, the first difference circuit calculates thevoltage V1=V(P2)−½V(P3), and the second difference circuit calculatesthe voltage V2=V(P2′)−½V(P3).

The scaling circuit preferably comprises a circuit for calculating thescaling factor; the circuit for calculating the scaling factorpreferably calculates the square root of the square sum of V1 and V2.This calculation is preferably implemented in analog, preferably from acircuit that first forms the squares of V1 and V2, then adds thesesquares and takes the square root of the sum. The root function can alsopreferably be implemented by a linear and/or square approximation.

The scaling circuit also preferably comprises a circuit for scaling theV1 and/or V2 values. This scaling preferably ensues using an analogprocessing that respectively divides V1 and/or V2 by the calculatedscaling factor.

The scaling circuit also preferably comprises a circuit that implementsthe allocation of the scaled V1 and/or V2 values to an orientation angleof the external magnetic field. This allocation preferably occurs usinga tabular allocation that can be implemented in analog and/or digital.This allocation can also preferably be integrated in an ASIC.

The output voltages V1 and V2 pending at the two sensor sub-circuits arealso preferably amplified in analog before the scaling factor iscalculated and the scaling is implemented. This measure is especiallyrecommendable given small output voltage signals. This measure also hasno diminishing influence on the scaling.

FIG. 5 shows a graph illustrating an embodiment of the inventive methodfor determining the orientation of an external magnetic field in thatthe temperature-dependency of the measurement can be eliminated byscaling with the scaling factor SQRT(V1 ²+V2 ²), and the angle of anexternal magnetic field can thus be measured independent of temperaturewithout having to use the resource-intensive arctan and/or arccotfunctions.

A vector V_(m)=(V1, V2) is entered in the orthogonal coordinate systemwith the V1 axis 5-1 and the V2 axis 5-2, where the vector components V1and V2 are the measured values from a sensor circuit as shown in FIG. 4.The dotted-line circle 5-5 represents all of the points (V1, V2) thatthe inventive sensor circuit can reach at a fixed temperature T1 byvarying the orientation of an external magnetic field. Since the GMRsensors in both sensor sub-circuits exhibit anti-parallel internalmagnetic orientation relative to one another and the sensor sub-circuitshave a magnetic orientation that is largely perpendicular to oneanother, the following applies:V 1=ΔR(T)×I×cos αandV 2=ΔR(T)×I×sin α

-   -   where α is the angle between the orientation of the external        magnetic field to be measured and the orientation of the GMR        sensor of the first sensor sub-circuit. Since the GMR sensors        have the same temperature dependency and the same current I        flows through the two sensor sub-circuits, the vector V_(m) and        the vector with the orientation of the external magnetic field        lie on one another. A unitary vector 5-4 that lies on the        unitary circle 5-6 is generated from V_(m)=(V1, V2) using a        scaling with SQRT(V1 ²+V2 ²):        V _(m0)=(V 1/SQRT(V 1 ² +V 2 ²),V 2/SQRT(V 1 ² +V 2 ²))=(sin α,        cos α)

All vectors (V1, V2) can be projected onto the unitary circle using thismethod so that a table with an allocation of (V1, V2) unitary circlevectors to the angle α suffices in order to be able to allocate an anglefor the external magnetic field to each (V1, V2) vector even given anunknown temperature.

FIG. 6 schematically shows the structure of an inventive embodiment ofthe electronic circuit with GMR sensors for determining the angle α forthe orientation of an external magnetic field. The sensor circuit 6-0has two sensor sub-circuits 6-1 and 6-2, where the sensor circuit 6-0preferably has a structure as in FIG. 4. The sensor circuit shown inFIG. 4 is particularly characterized by the anti-parallel internalmagnetic orientation of the first and second GMR sensors within a sensorsub-circuit as well as by the vertical magnetic orientation of the twosensor sub-circuits relative to one another. Furthermore, the four GMRsensors preferably have largely the same fundamental resistance R0(T)and the same magneto-resistive resistance ΔR(T) as well as the sametemperature dependency of the two parameters. Finally, the currentthrough the first sensor sub-circuit 4-2 is preferably of the same sizeas the current through the second sensor sub-circuit 4-3. The currentultimately flows off in the current sink 4-12.

The difference circuit 6-3 of the first sensor sub-circuit 4-2 and thedifference circuit 6-2 of the second sensor sub-circuit 4-3 respectivelycalculate the voltage differences between the sensor voltages V(P1),V(P2) and V(P3) or, respectively, V(P1), V(P2′) and V(P3), as shown inFIG. 4, so that the difference voltages V1 and V2 are generated:V 1=½ΔR(T)cos αV 2=½ΔR(T)sinα

-   -   where α is the angle between the orientation of the external        magnetic field 4-13 and the internal orientation of the first        GMR sensor of the first sensor sub-circuit 4-14.

The difference voltages V1 and V2 are further-processed by theembodiment of the inventive electrical scaling circuit 6-5 shown in FIG.6: V1 and V2 are first squared in parallel in the analog circuits 6-6and 6-7 for squaring and the squared values are summed in an analogadder 6-8. Subsequently, the root of the square sum is taken in acircuit 6-9 for taking the square root. The circuit 6-9 for taking thesquare root is preferably an analog circuit. The output signal of thecircuit for calculating the scaling factor is the scaling factor SQRT(V1²+V2 ²) that is introduced into the two dividers 6-10. An unscaled V1and V2 value respectively exist simultaneously at inputs of the twodividers 6-11 and 6-12, so that a scaling of V1 or of V2 can beimplemented:V 1 _(Sk) =V 1/SQRT(V 1 ² +V 2 ²)=cos α  (11)andV 2 _(sk) =V 2/SQRT(V 1 ² +V 2 ²)=sin α  (12)

The scaled values are thus temperature-independent. The angle α can beunambiguously determined from them.

For the purposes of promoting an understanding of the principles of theinvention, reference has been made to the preferred embodimentsillustrated in the drawings, and specific language has been used todescribe these embodiments. However, no limitation of the scope of theinvention is intended by this specific language, and the inventionshould be construed to encompass all embodiments that would normallyoccur to one of ordinary skill in the art.

The present invention may be described in terms of functional blockcomponents and various processing steps. Such functional blocks may berealized by any number of hardware and/or software components configuredto perform the specified functions. For example, the present inventionmay employ various integrated circuit components, e.g., memory elements,processing elements, logic elements, look-up tables, and the like, whichmay carry out a variety of functions under the control of one or moremicroprocessors or other control devices. Similarly, where the elementsof the present invention are implemented using software programming orsoftware elements the invention may be implemented with any programmingor scripting language such as C, C++, Java, assembler, or the like, withthe various algorithms being implemented with any combination of datastructures, objects, processes, routines or other programming elements.Furthermore, the present invention could employ any number ofconventional techniques for electronics configuration, signal processingand/or control, data processing and the like.

The particular implementations shown and described herein areillustrative examples of the invention and are not intended to otherwiselimit the scope of the invention in any way. For the sake of brevity,conventional electronics, control systems, software development andother functional aspects of the systems (and components of theindividual operating components of the systems) may not be described indetail. Furthermore, the connecting lines, or connectors shown in thevarious figures presented are intended to represent exemplary functionalrelationships and/or physical or logical couplings between the variouselements. It should be noted that many alternative or additionalfunctional relationships, physical connections or logical connectionsmay be present in a practical device. Moreover, no item or essential tothe practice of the invention unless the element is specificallydescribed as “essential” or “critical”. Numerous modifications andadaptations will be readily apparent to those skilled in this artwithout departing from the spirit and scope of the present invention.

1. A method for determining an orientation of an external magnetic fieldwith giant magneto-resistor (GMR) sensors, comprising: a) providing asensor circuit comprising at least a first and a second sensorsub-circuit that are connected parallel to one another, the first andthe second sensor sub-circuit each respectively comprising at least afirst and a second GMR sensor that are connected in series and that arerespectively arranged in opposite internal magnetic orientation, theinternal magnetic orientation of the GMR sensors of the first sensorsub-circuit being essentially perpendicular to the internal magneticorientation of the GMR sensors of the second sensor sub-circuit, thesensor circuit being fed by a current source, and draining through acurrent sink; b) obtaining three respective sensor voltages from a firstpoint, a second point, and a third point in the first and the secondsensor sub-circuit, the first point lying on a connecting line from thecurrent sink to the second GMR sensor, the second point lying on aconnecting line from the second GMR sensor to the first GMR sensor, andthe third point lying on the connecting line from the first GMR sensorto the current supply; c) generating a first difference voltage using afirst difference circuit from the three sensor voltages of the firstsensor sub-circuit, and generating a second difference voltage from asecond difference circuit from the three sensor voltages of the secondsensor sub-circuit; d) calculating a scaling factor from the firstdifference voltage and the second difference voltage; and e) determiningthe orientation of an external magnetic field by scaling the firstdifference voltage and the second difference voltage with the scalingfactor.
 2. The method according to claim 1, wherein current through thefirst sensor sub-circuit and current through the second sensorsub-circuit are substantially the same.
 3. The method according to claim1, wherein the first difference circuit and the second differencecircuit have a substantially identical structure.
 4. The methodaccording to claim 1, wherein the scaling factor is the square root ofthe square sums of the first difference voltage and the seconddifference voltage or an approximation to this equation.
 5. The methodaccording to claim 4, further comprising providing an analog circuitconfigured to calculate squares of at least one of the first differencevoltage and the second difference voltage.
 6. The method according toclaim 4, further comprising providing an analog circuit configured tocalculate the square sum of the first difference voltage and the seconddifference voltage.
 7. The method according to claim 4, furthercomprising approximating the calculation of the square root of thesquare sum of the first difference voltage and the second differencevoltage with an analog circuit.
 8. The method according to claim 1,further comprising scaling at least one of the first difference voltageand the second difference voltage by dividing the respective differencevoltage by the scaling factor.
 9. The method according to claim 1,further comprising calculating orientation angles of the externalmagnetic field utilizing at least one of scaled first difference voltagevalues and scaled second difference voltage values.
 10. A systemutilizing GMR sensors for determining the orientation of an externalmagnetic field, comprising: a sensor circuit having a first and a secondsensor sub-circuit, the first sensor sub-circuit being connected inparallel to the second sensor sub-circuit and the first and the secondsensor sub-circuit each respectively comprise at least a first and asecond GMR sensor that are connected in series and that are respectivelyarranged in opposite internal magnetic orientation relative to oneanother, the internal magnetic orientation of the GMR sensors of thefirst sensor sub-circuit is substantially perpendicular to the internalmagnetic orientation of the GMR sensors of the second sensorsub-circuit, the sensor circuit being fed by a current source, anddraining through a current sink; wherein the first and the second sensorsub-circuit respectively comprise a first point, a second point, and athird point, the first point lying on a connecting line from the currentsink to the second GMR sensor, the second point lying on a connectingline from the second GMR sensor to the first GMR sensor, and the thirdpoint lying on a connecting line from the first GMR sensor to thecurrent supply, so that the three sensor voltages can be respectivelytaken at the three points; the system further comprising a first and asecond difference circuit, the first difference circuit configured togenerate a first difference voltage from the three sensor voltages ofthe first sensor sub-circuit, and a second difference circuit beingconfigured to generate a second difference voltage from the three sensorvoltages of the second sensor sub-circuit; and a scaling circuitconfigured to read out the first difference voltage and the seconddifference voltage and to scale them such that the orientation of theexternal magnetic field can be determined from the scaled values. 11.The system according to claim 10, wherein current through the firstsensor sub-circuit and current through the second sensor sub-circuit aresubstantially the same.
 12. The system according to claim 10, whereinthe first and the second differential circuit are substantiallyidentical.
 13. The system according to claim 10, wherein the scalingcircuit comprises a circuit for calculating the scaling factor.
 14. Thesystem according to claim 13, wherein the scaling circuit comprises acircuit configured to calculate the square root of the square sum of thefirst difference voltage and the second difference voltage values or anapproximation to this equation.
 15. The system according to claim 10,wherein the scaling circuit comprises a circuit configured to scale thefirst difference voltage and the second difference voltage values. 16.The system according to claim 15, wherein the circuit for scaling thefirst difference voltage and the second difference voltage valuescomprises a circuit configured to divide the first difference voltageand the second difference voltage values by the scaling factor.
 17. Thesystem according to claim 10, further comprising at least one of analgorithm and data configured to calculate an angle for the orientationof the external magnetic field utilizing at least one of the scaledfirst difference voltage and second difference voltage values.
 18. Thesystem according to claim 10, wherein the scaling circuit comprises acircuit configured to calculate a scaled first difference voltage andsecond difference voltage value pair utilized to determine anorientation angle of the external magnetic field.
 19. The systemaccording to claim 10, wherein at least one circuit of the scalingcircuit is integrated in an ASIC.
 20. The system according to claim 10,further comprising a pre-amplification mechanism for at least one of thefirst difference voltage and second difference voltage.